1. Field of the Invention
The present invention relates to an optical fiber for wavelength division multiplexing (WDM) communications, and more particularly, to an optical fiber having a profile which optimizes chromatic dispersion in a desired wavelength band by adjusting the refractive index of an optical waveguide.
2. Description of the Related Art
In general, WDM communications transmits a plurality of optical signals having different wavelengths via an optical fiber, and can effectively use the low loss characteristics of an optical fiber over a wide wavelength band. Thus, at present, communications techniques using WDM are being used more widely.
It is known that an optical fiber, which is the most widely used in the above WDM communications, must have a wide communications region to transmit a signal having a 1550 nm wavelength band. As an advantage, an optical fiber having a communications region in the wavelength band of 1550 nm has low attenuation, since light in the wavelength band of 1550 nm is attenuated the least when propagating the core of a optical fiber according to the characteristics of a silica optical fiber. Accordingly, when communications is made using an optical fiber having a transmission region capable of transmitting the 1550 nm wavelength band signal, repeaters can be spaced widely apart, thereby drastically reducing costs for maintenance and repair for long distance transmission.
As another advantage, optical communications of the 1550 nm wavelength band signal can amplify an optical signal using an optical fiber itself. Such optical amplification can not only reduce the size of an external amplifier but also does not require an external generator, so that the size of a transmission system can be remarkably diminished as well.
For example, a dispersion shifted fiber (DSF) is commonly used of late as an optical fiber having a communications region of the 1550 nm wavelength band. The DSF, having chromatic dispersion of "0" in a wavelength of 1550 nm, is effectively used to transmit a 1550 nm-wavelength signal. A representative example of such a DSF is disclosed in U.S. Pat. No. 4,715,679 entitled "Low-dispersion Low- Attenuation Single Mode Optical Waveguide" by Venkata A.Bhagavatula.
Also, an optical amplifier uses a rare earth ion doped fiber formed by doping a rare earth material to the core of an optical fiber. The rare earth material is generally erbium, neodymium or praseodymium. An erbium doped fiber amplifier (EDFA), where erbium among the rare earth materials is doped to the core, is the most widely used, and allows an optical communications network to be realized.
The EDFA has characteristics representing a maximum gain at a wavelength band of 1530 nm, and it is known that a wavelength band capable of using the amplification effect is usually, approximately between 1530 nm and 1565 nm. As to a current optical fiber and an optical communications technique using the same, much effort is made to widen a wavelength band which can sufficiently utilize the amplification properties of the EDFA and to flatten the difference between gains over wavelength bands.
At present, single mode optical fibers for WDM communications are manufactured so that the inclination of chromatic dispersion and a zero dispersion wavelength are suitable for a wavelength of 1550 nm. However, in this case, this WDM communications single mode fiber is limited with respect to multi-channel transmission because of nonlinear effects such as four-wave mixing, and the multichannel transmission is more difficult since the wavelength band of the optical fiber is not equal to a gain wavelength region of the EDFA. Also, in the optical fibers having a zero dispersion wavelength set to 1550 nm, the inclination of the chromatic dispersion is not consistent with the gain wavelength band of the EDFA. As described above, the optical fibers having a zero dispersion wavelength cause a limit where transmission quality is precipitously deteriorated even when there is little change in the inclination of chromatic dispersion.
In order to suppress an increase in the aforementioned nonlinear effect, an optical fiber must have the characteristics in which zero dispersion is not made in the gain wavelength band of the EDFA by controlling chromatic dispersion of the optical fiber. That is, the optical fiber is designed to have characteristics in which zero dispersion is not made in a wavelength region of between 1350 nm and 1560 nm, in order to minimize or remove the nonlinear effect such as the four-wave mixing in the gain wavelength region of the current EDFA.